The genus pestivirus belongs to the family of Flaviviridae and consists of Classical Swine Fever Virus (or hog cholera virus), which is the causative agent of classical swine fever bovine viral diarrhoea virus (BVDV) infectious to cattle and border disease virus (BDV) infectious to sheep.
Pestiviruses are small enveloped RNA viruses having a diameter of 40-60 nm. The virions consist of a nuclear capsid enveloped by a lipid layer embedded with glycoproteins. The genome of pestiviruses consists of a single stranded RNA of approximately 12.5 kB. It contains a single open reading frame (ORF) which is flanked by parts that are not translated. The 3' end is not polyadenylated.
Viral proteins are formed by co- and posttranslational processing of a hypothetic polyprotein, while the structural proteins are coded in the 5' end of the genome. In contrast to other flaviviridae the pestiviruses have a sequence coding for a non-structural p23 protein at the 5' end. This N-terminal protease (Npro) is spliced from the next protein by an autoproteolytical process. In the 3' direction the sequence coding for a p14 nuclear capsid and a signal sequence, which is responsible for the translocation of the subsequent sequences for glycoprotein E0, E1 and E2 in the lumen of the endoplasmatic reticulum (ER), are following. The splicing of the single glycoproteins probably is caused by cellular signalases in the ER. The glycoproteins can form complex structures in infected cells by dimensioning through S-S-bridges. The function of these complex structures is not yet known.
Next to the sequences for the structural proteins the sequences for the non-structural proteins p125, p10, p30 and p133 are found in the polyprotein. Analogous to posttranslational processes in cytopathogenic Bovine Viral Diarrhea Virus (BVDV, another pestivirus) anp 80 protein can be detected after processing the p125 protein (Desport, M. and Brownlie J., Arch. Virol. Suppl. 3, 261-265, 1991). The p133 non-structural protein, which is processed into p58 and p75 proteins, contains sequence motifs which resemble RNA polymerase sequences. The amino acid sequences of the non-structural CSFV proteins are considered to be approximately: 3-1142 (p125), 1143-1206 (p10) shown in SEQ ID NO: 1 and 2 herein. The putative N-terminus of p80 is amino acid no. 460 (SEQ ID NO: 1 and 2). The start of p30 is also shown in SEQ ID NO. 1 and 2, i.e. amino acid number 1207. The complete sequence of p30 for CSFV Alfort shown in Meyers et al. (Virology 171, 555-567, 1989; FIG. 4 amino acids 2337-2683). The DNA sequences encoding these proteins are also shown in SEQ ID NO. 1 herein and in Meyers et al. (supra).
On infection with CSFV acute, peracute, chronic or clinical invisible symptoms can occur. The severity of the disease is dependent on the infectious load on one hand, and on the other hand the age of the animal, the immune competence and total constitution of the animal form important factors.
In the peracute illness, which results in the death of the animal after three to five days after infection with a highly virulent strain (e.g. the CSFV-Brescia strain), only a fever is observable. In the acute disease state, next to a high temperature, leucopenia, conjunctivitis and loss appetite are observable. In the final stage of the disease central nervous system disturbances occur. Further characteristics are an atrophy of the thymus, cyanosis of the skin and cutaneous haemorrhages, partially caused by a thrombocytopenia. Mortality of the disease in the acute phase is 30-100%.
The chronic disease form is found after infection with mesogenic virus strains. This form is most dangerous when piglets are infected in utero by diaplacental infection. After birth these piglets only survive for 6-8 weeks during which time they form a source of infection.
Pigs that survive a postnatal infection, gain lifetime immunity, which probably results from an induction of the humoral immune response. Neutralising antibodies are found after two to three weeks after infection.
Several vaccines have been developed for this economically important disease. Vaccines with inactivated virus give only shortlasting protection and are not used anymore (Biront, P. and Leunen, J., in: Liess, B. (ed.): Classical swine fever and related viral infections. Martinus Nijhoff Publ., Boston pp. 181-200, 1988). Better protection has been established with attenuated viruses obtained by serial passaging in rabbits (C-strains) or in cell cultures (e.g. Thiverval-strain) (Launais, M. et al., Rev. Med. Vet., 123, 1537-1554, 1972; Shimizu Y., Jap. J. Trop. Agr. Res. Sci., 13, 167-170, 1980).
However, a disadvantage of these vaccines is that they do not discern between vaccinated animals and animals infected with a field virus. In the European Community, therefore, use of these vaccines is forbidden presently and control of classical swine fever is established by isolation and slaughtering of infected swine.
In recent years some results have been obtained with live vaccines based on recombinant viruses (van Zijl, M. et al., J. Virol. 65, 2761-2765, 1991; Rumenapf. T. et al., J. Virol. 65, 589-597, 1991; Hulst, M. M. et al., J. Virol. 67, 5435-5442, 1993). These vaccines have all been based on the expression of structural glycoproteins in recombinant vector viruses.